Total joint arthroplasty has substantially improved the quality of life for millions of people, yet a significant number of joint replacements fail each year, particularly "revision" joint replacements (i.e., replacements inserted after failure of the "primary" or original replacement). The basic objective of the work proposed is to investigate the pathomechanics of revision failure, focusing on the biologic characteristics of bone and soft tissue and the effects of implant motion and particulate debris. The role of bone grafting will also be studied. Four hypotheses will be tested: 1): the density of foreign body giant cells in a revision cavity depends on the size of the polyethylene particles in the model; 2) bone ingrowth and implant fixation are greater for a primary implant cavity than a revision cavity, both with and without implant motion; 3) bone grafting improves implant fixation and bone ingrowth in a revision cavity, with and without motion; and 4) when bone grafting is used, implant fixation and bone ingrowth are superior for a primary cavity than for a revision cavity. Soballe implants will be inserted into the femoral condyles of 28 adult mongrel dogs (the unstable Soballe implant provides a controlled amount of axial motion; the stable Soballe implant does not move). The study will proceed as follows: in Year 1, the study of the particle size on membrane and bone characteristics will be completed. This ongoing experiment shows the applicability of the Soballe implant for generating the essential features of a loosened cemented implant (i.e., revision cavity). In Years 2-3, the effects of implant motion and bone grafting in this revision cavity will be investigated. The following conditions will be studied: stable primary, unstable primary, stable revision, unstable revision, stable primary with bone graft, unstable primary with bone graft, stable revision with bone graft, and unstable revision with bone graft. At euthanasia, anteroposterior and lateral radiographs will be obtained and bacterial cultures will be taken, and the block of bone and membrane surrounding the implant will be removed. Analyses and intergroup comparisons (Years 4-5) will be conducted for implant/interface shear strength, fibrous membrane thickness, bone ingrowth, and presence of macrophages, fibroblasts, synovial cells, foreign body giant cells, and debris presence. The results of these studies are intended to provide quantitative data on the differences between primary and revision joint replacement, with and without bone graft, and indicate directions for improving the clinical success rate of revision joint replacement.